1. Field of the Invention
The invention relates to the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425 C-600 C, usually 460 C-560 C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500 C-900 C, usually 600 C-750 C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. Although there are myriad individual variations, typical designs are shown in U.S. Pat. No. 3,849,291 (Owen) and U.S. Pat. No. 3,894,934 (Owen et al), and U.S. Pat. No. 4,368,114 (Chester et at.) which are incorporated herein by reference.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units like this are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many recent patents. The H.E.R. design is used in most new units because it permits operation of an FCC with less catalyst inventor (and less catalyst loss), and because such units tend to have both less CO emissions and less NOx emissions than the single dense bed regenerators.
Unfortunately, it has not been economically justifiable to convert older style, single dense bed regenerators to the modern H.E.R. design because of the high capital cost associated with simple scrapping the old single bed regenerator. Attempts to simply use the old single stage regenerator as part of a modern two stage, H.E.R. design have not been too successful, as the old single stage units are much larger than either of the beds in an H.E.R. unit. Another complication has been that many of the older units were not designed to operate at the higher temperatures associated with complete CO combustion.
Rather than scrap older FCC regenerators, refiners have tried to improve them. Several regenerator designs have been proposed which could perhaps use some existing equipment, and which provide a fast fluidized bed coke combustor operating largely or entirely within a bubbling fluidized bed.
Gross U.S. Pat. No. 4,118,338, which is incorporated herein by reference, teaches a coke combustor vessel immersed in a bubbling dense bed. The coke combustor has an inverted cone section at the base, for admission of spent catalyst and for relatively large amounts of recycled regenerated catalyst. The coke combustor is suspended in the bubbling dense bed, and relies on fluid dynamics to circulate regenerated catalyst from the bubbling dense bed back into the base of the coke combustor, along with spent catalyst. The driving force presumably is the difference in densities--the density of the catalyst in the bubbling dense bed is usually roughly double that of the somewhat dense phase of catalyst in the coke combustor. Recycled to spent catalyst ratios of "from a small fraction (&lt;0.5) to a high multiple of the catalyst flow (&gt;10)."
Gross et al U.S. Pat. No. 4,448,753, which is incorporated herein by reference, is directed to a coke combustor somewhat submerged in a bubbling dense bed. The coke combustor operates conventionally, i.e., with large amounts of regenerated catalyst recycle to the coke combustor. The patent teaches how to avoid flow reversal with high catalyst recycle rates, 10 tons recycled per ton of spent catalyst--see the middle column of Table 2. Although heat transfer could theoretically occur, there is so much catalyst recycle that delta T between the coke combustor and the second dense bed will be very low, reducing the amount of heat transfer by indirect heat exchange that could occur. Such a design will usually require new construction to handle increased catalyst traffic produced by large amounts of catalyst recycle.
I wanted a way to have high efficiency regeneration of catalyst in a way that could readily be performed in an existing single dense bed regenerator, without catalyst recycle. I wanted to avoid the power consumption of conventional catalyst recycle operations. There is a lot of energy consumed in lifting tons per minute of catalyst up 10 or 20 feet into a bubbling dense bed and then dumping this catalyst back down into a coke combustor.
The recycle of large amounts of hot regenerated catalyst into the coke combustor will also increase apparent catalyst traffic in the regenerator. In general, particulates emissions are proportional to catalyst traffic, and doubling of catalyst traffic will double particulates emissions. This is not too difficult to handle in new construction, such as a new high efficiency regenerator, which operates with large amounts of catalyst recycle, and is sized, and has cyclones adapted to handle the catalyst traffic. In older units, such as bubbling dense bed regenerators, it is more difficult to modify the unit to handle increased catalyst traffic.
There has been little work done in this area. Most refiners simply recycle catalyst, and pay the cost of power consumption and increased particulates emissions. Part of the reason for this acceptance of catalyst recycle is that conventional approaches do not provide enough surface area for effective heat transfer. U.S. Pat. No. 3,923,686, filed in 1972, before the benefits of catalyst recycle were apparent to refiners, shows in FIG. 3 a somewhat immersed coke combustor is disclosed, which could be heated to a limited extent by heat transfer across a common wall shared with a bubbling dense bed regenerator.
A more effective heat transfer method was disclosed in Johnson U.S. Pat. No. 2,401,739. This teaches a multistage regenerator, where the first stage of regeneration ran hotter than the second. "In . . . my process it is preferred to maintain the initial regeneration zone 32 at a higher temperature than the final regeneration zone . . . Operating in this manner, heat is conducted from the initial regeneration zone to the final regeneration zone through the walls separating the two zones." Column 4 Lines 50-59. Johnson recognized that heat transfer could occur through regenerator vessel walls, but used this to cool the first stage of regeneration, rather than heat it. He also avoided CO combustion, and lacked a dilute phase transport riser which would have permitted safe CO combustion.
To summarize, the art has embraced high efficiency regenerator with heated coke combustors, but generally uses catalyst recycle to fire up the coke combustor. This approach works well in new construction, but not in existing units, and it also consumes a lot of horsepower for catalyst recirculation, and increases catalyst traffic in the regenerator. Some of the earliest work on coke combustors, operating without catalyst recycle, may have had modest amounts of indirect heat exchange, but based on the Figures shown, not enough to permit effective heating of a coke combustor.
I discovered a way to overcome many of the deficiencies of the prior art methods of regenerating spent FCC catalysts in a single dense bed by, in effect, immersing a two stage regenerator in the space previously occupied by a single stage regenerator. My design also permits reduction of NOx emissions from a conventional, single dense bed FCC regenerator by providing a practical way to achieve two stages of regeneration in an existing, single stage or single bed regenerator. I use the shell of the conventional single stage regenerator, and much of the equipment heretofore associated with it, to economically get two stages of regeneration in a vessel previously used to contain a single dense bed of FCC catalyst for regeneration. This approach permits the regenerator to accommodate more difficult stocks, and to tolerate somewhat higher temperatures.